In general, electromagnetic wave receivers often require an automatic gain control (AGC) to process signals that exhibit large amplitude variations. Digital receiving systems, in particular, require AGC circuitry due to quantization noise and distortion in their analog-to-digital converters (ADCs). This AGC requirement is rather critical in radar intercept receivers. In these receivers, large bandwidths necessitate very high speed sampling to meet the Nyquist criterion which entails a reduced number of effective bits, signal-to-noise-and-distortion (SINAD) ratio and dynamic range. Commercial ADCs sampling at 200 million samples/s, for example, provide 7 effective bits which amounts to a SINAD of 44 dB. Assuming that 14 dB of the SINAD ratio is required for signal detection and processing, this leaves 30 dB of dynamic range. When undersampling is used, i.e. when an ADC samples a signal at an intermediate frequency higher than half its sampling rate, it is typical then for the ADCs to provide only 6 or 5 effective bits. The dynamic range obtained without an AGC, consequently, falls well short of the 60 dB or more required in radar intercept receivers.
A conventional AGC circuit for digital radar intercept receivers has an analog IF input signal applied to a video detector whose output is applied to a track-and-hold circuit that samples-and-holds the amplitude of the signal starting at the leading edge of each pulse as demarked by a leading edge trigger (LET) circuit. This sampled and held amplitude is compared to reference levels in control circuitry which provides a signal to adjust a programmable attenuator in accordance with the level of that amplitude. The analog IF input signal is applied to the programmable attenuator via an analog delay line where the signal is attenuated to a suitable level before being digitized by a sampling ADC. By delaying the pulse in an analog delay line, the fast programmable attenuator has time to settle before the arrival of the delayed pulse. This AGC circuit, as a result, operates on a pulse-by-pulse basis. The sampling ADC circuit is started and stopped by the control circuitry based on the pulse LET, a pulse trailing edge trigger (TET) circuit and the time delay of the analog delay line. The type of conventional AGC circuit is described in more detail in U.S. Pat. No. 5,161,170 by Paul H. Gilbert et al.
The available analog delay lines in the conventional AGC described above is one source of problems associated with that circuit. This results in these AGC circuits being bulky, expensive, unreliable and to require individual trimming or calibration for interoperability between radar intercept receivers. Present analog cable delay lines entail a large and heavy coil of cable for the typical time delays required. Repeaters may be inserted at regular intervals in the cable to reduce signal attenuation but this is at the expense of bulk and reliability.
Available surface acoustic wave (SAW) delay lines have a limited bandwidth, an insertion loss of the order of 30 dB and produce significant signal distortions. These distortions will reduce interoperability between receivers, at least unless the SAW devices are individually trimmed to distort the signal alike which incurs additional costs. The optical delay lines presently available reduce the dynamic range due to limitations of available photo-detectors.
Another drawback of this conventional AGC circuit is that the time delay of the analog delay lines is fixed. If the rise time of a radar pulse exceeds that time delay, for instance, the control circuitry will not detect the true peak of the pulse and may select insufficient attenuation and the pulse upon reaching the ADC may cause an over-range. The same may also happen if the pulse exhibits amplitude variation, whether intentional or unintentional.